Abstract

Internal noise and sampling efficiency are the main factors which limit visual performance. In a previous study [Vis. Res. 43 (2003) 1103] we compared the variance of human reaction time to that of an ideal observer and found that the sampling efficiency to suprathreshold stimuli was much lower than that obtained in detection experiments. In order to bypass the effects of the motor system on visual performance, we used a flash–sound simultaneity paradigm. We found that the sampling efficiency for 0.4- and 4-c/deg near-threshold Gabor patches is higher only by a factor of 2.5 than that to above-threshold patterns. The signal-dependent multiplicative internal noise was similar to the additive internal noise at lower signal contrast levels and exceeded it at higher signal contrast levels. The results show that real observers’ performance for detecting suprathreshold stimuli can be accounted for by a model taking into account the non-linear visual–signal transduction and multiplicative components of the internal noise induced by the signal and external noise. In addition, this model assumes that performance depends on the response duration, rather than signal duration. The results imply that the multiplicative internal noise induced by high contrast visual signals determines performance for suprathreshold visual detection.